It is known to cast metal strip by continuous casting in a twin roll caster. In such a process, molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product which is delivered downwardly from the nip between the rolls. The molten metal may be introduced into the nip between the two rolls via a tundish and a metal delivery nozzle system located beneath the tundish so as to receive a flow of metal therefrom and to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in engagement adjacent the ends of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
Twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, for example aluminum. However, there have been problems in applying the technique to the casting of ferrous metals. One particular problem has been the propensity for ferrous metals to produce solid inclusions which clog the very small metal flow passages required in a twin roll caster.
The use of silicon-manganese in ladle deoxidation of steel was practiced in ingot production in the early days of Bessemer steelmaking. As a result, the equilibrium relations between the reaction product molten manganese silicates and the residual manganese, silicon and oxygen in solution in steel are well known. However in the development of technology for the production of steel strip by slab casting and subsequent cold rolling, silicon/manganese deoxidation has generally been avoided and it has been generally considered necessary to employ aluminum killed steels. In the production of steel strip by slab casting and subsequent hot rolling followed often by cold rolling, silicon/manganese killed steels produce an unacceptably high incidence of stringers and other defects resulting from a concentration of inclusions in a central layer of the strip product.
In the continuous casting of steel strip in a twin roll caster, it is critically important to generate a finely controlled flow of steel at constant velocity along the length of the casting rolls to achieve sufficiently rapid and even cooling of steel over the casting surfaces of the rolls. This requires that the molten steel be constrained to flow through very small flow passages in refractory materials in the metal delivery system under conditions in which there is a tendency for solid inclusions to separate out and clog those small flow passages.
After an extensive program of strip casting various grades of steel in a continuous strip roll caster, it has been determined that conventional aluminum killed carbon steels or partially killed steel with an aluminum residual content of 0.01%, or greater, generally cannot be cast satisfactorily because solid inclusions agglomerate and clog the fine flow passages in the metal delivery system to form defects and discontinuities in the resulting strip product. This problem can be addressed by calcium treatment of the steel to reduce the solid inclusions, but this is expensive and needs fine control adding to the complexity of the process and equipment. On the other hand, it has been found that it is possible to cast strip product without stringers and other defects normally associated with silicon/manganese killed steels because the rapid solidification achieved in a twin roll caster avoids the generation of large inclusions and the twin roll casting process results in the inclusions being evenly distributed throughout the strip rather than being concentrated in a central layer. Moreover, in thin strip casting, it is possible to adjust the silicon and manganese contents so as to produce liquid deoxidation products at the casting temperature to minimize agglomeration and clogging problems.
In conventional silicon/manganese deoxidation processes, it has not been possible to lower free oxygen levels in the molten steel to the same extent as is achievable with aluminum deoxidation, and this problem in turn has inhibited desulfurization. For continuous strip casting, it is desirable to have a sulfur content of the order of 0.009% or lower. In conventional silicon/manganese deoxidation processes in the ladle, the desulfurization reaction is very slow, generally more than an hour, and it becomes impractical to achieve desulfurization to such low levels particularly in the case where the steel is produced by the EAF route using commercial quality scrap. Such scrap may typically have a sulfur content in the range 0.025% to 0.045% by weight. Details relating to strategies for enabling effective and efficient deoxidation and desulfurization of silicon/manganese killed steel, and for refining of high sulfur silicon/manganese killed steel to produce low sulfur steel which has free oxygen levels suitable for continuous thin strip casting, are disclosed in co-pending U.S. patent application Ser. No. 60/280,916, which is assigned to the assignee of the present invention, and the disclosure of which is expressly incorporated herein by reference.
When casting thin steel strip in a twin roll caster the molten steel in the casting pool will generally be at a temperature of the order of 1500° C. and above, and it is therefore necessary to achieve very high cooling rates over the casting surfaces of the rolls. It is particularly important to achieve high heat transfer and extensive nucleation on initial solidification of the steel on the casting surfaces to form the metal shells. U.S. Pat. No. 5,720,336 describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry such that a substantial proportion of the metal oxides formed as deoxidation products are liquid at the initial solidification temperature so as to form a substantially liquid layer at the interface between the molten metal and each casting surface. It has been determined that nucleation is also dependent on the presence of oxide inclusions in the steel melt and that surprisingly it is not advantageous in twin roll strip casting to cast with “clean” steel in which the number of inclusions formed during deoxidation has been minimized.
Steel for continuous casting is subjected to deoxidation treatment in the ladle prior to casting as described hereinabove. In twin roll casting the steel is generally subjected to silicon manganese ladle deoxidation although it is possible to use aluminum deoxidation with calcium addition to control the formation of solid Al2O3 inclusions that can clog the fine metal flow passages in the metal delivery system through which molten metal is delivered to the casting pool. It has been determined that while lowering the steel oxygen level of unrefined molten steel allows for subsequent desulfurization thereof as described hereinabove, it undesirably reduces the volume of oxide inclusions. If the total oxygen content of the steel is reduced below a certain level, the nature of the initial contact between the steel and roll surfaces can be adversely effected to the extent that there is insufficient nucleation to generate rapid initial solidification and high heat flux. Following desulfurization, free oxygen is therefore injected into the molten steel to raise its free oxygen content to a level that promotes sufficient nucleation to generate rapid initial solidification of the molten steel onto the casting rolls and production of a satisfactory strip product. As a result of the reoxidation of the molten steel, it then contains a distribution of oxide inclusions (typically MnO, CaO, SiO2 and/or Al2O3) sufficient to provide an adequate density of nucleation sites on the roll surfaces for initial solidification and the resulting strip product exhibits a characteristic distribution of solidified inclusions. Details relating to one strategy for injecting oxygen into a ladle of steel prior to casting thereof are set forth in co-pending U.S. patent application Ser. No. 60/322,261, which is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference.
While the above-referenced patent applications disclose systems and strategies for carrying out deoxidation, desulfurization and reoxidation steps in the ladle refinement of steel prior to casting into steel strips, these processes tend to require tedious techniques for determining the process parameters required to achieve the refined steel. For example, to reduce the percentage of sulfur in the molten steel to a desired percentage, a controllable quantity of flux must be added thereto. As another example, the melting point of inclusions in the reined molten steel must be below a threshold temperature to ensure that a substantially liquid oxide layer exists at the interface between the molten metal and each casting roll surface. The total amount of free oxygen added in the reoxidation step, as well as the amount and composition of flux and/or alloy additions, must therefore be known and controlled to provide for a desired inclusion melting temperature in the batch or ladle of refined steel. Finally, it is necessary from a castability standpoint to determine the inclusion melting temperature of the batch of refined steel to determine whether the ladle may be routed to the strip casting process or whether it requires re-working in order to adjust the inclusion melting temperature. What is therefore needed is a strategy for determining these various process parameters for the ladle refinement of steel, wherein such strategy is straightforward in its application, easily implemented in software, and readily adaptable to a continuous steel strip casting process.